The present invention relates to a system for mounting an inboard marine engine to the hull of a boat and to a fluid mount construction for use in such a mounting system.
Resilient vibration and shock absorbing mounts have long been used between inboard marine engines and the boat hulls to which they are attached. Such engine mounting arrangements have included systems in which each of several separate mounts includes a flexible solid elastomer element, or a combination of elastomer and rigid or semi-rigid mounts.
U.S. Pat. No. 3,259,099 discloses a three point mounting system for an inboard marine engine in which each of the mounts includes a hollow cylindrical elastomer member mounted between cylindrical inner and outer metal sleeves. One of the sleeves is attached to the engine and the other to the boat hull or an intermediate supporting structure. Thus, solid elastomer elements help isolate the transmission of vibration from the engine to the boat hull and to cushion the impact of shock loads occurring, for example, by the boat travelling over rough water. In the particular mounting arrangement shown, one resilient mount is located centrally in front of the engine and the other two are spaced laterally on either side of the engine at the rear thereof.
In U.S. Pat. No. 3,865,068 another type of three point mounting system is disclosed in which a pair of lateral rigid mounting members are located approximately mid-engine on either side thereof and the rear of the engine is attached to the boat transom by a radially expandable elastomeric element between the engine drive shaft housing and a mounting hole through the transom.
U.S. Pat. No. 4,717,130 describes a shock and vibration dampening suspension system for mounting an inboard marine engine and its attached outboard drive unit to a boat. Pairs of forward and rear solid elastomer mounts are utilized with each pair comprising a varying construction to accommodate lifting forces at the front of the engine and opposite downwardly acting forces at the rear. The solid elastomer members are of a block-like construction and are bonded to rigid upper and lower connecting members attached to the engine and the engine bed on the boat hull.
Solid elastomer mounts of other shapes, sizes and locations have been used to support inboard marine engines on the boat hull and to dampen the transmission of vibrations from the former to the latter. The elastomer mounts also serve to cushion to some extent light shock loadings, such as are encountered in normal operation.
As with any engine, the frequency of vibrations transmitted by the engine to the boat (and of course experienced by the passengers) varies substantially from idle or low speed to cruising and high speed operation. Ideally, optimum isolation of vibratory forces would require varying stiffnesses of elastomer elements for varying vibration frequencies. This is, of course, impractical or impossible and, therefore, marine engine mounts have typically utilized an elastomer material which represents a compromise in stiffness and optimum vibration dampening effectiveness. Similarly, the capability of the elastomer to absorb or cushion light or low shock loads is compromised as well.
For use in the automotive industry, a fluid mount has been developed which utilizes the combined damping and isolation features of a solid elastomer and a hydraulic cushioning device. In such a fluid mount, a solid annular elastomer element is disposed between and bonded to two rigid connector members. One of the connector members is attached to the engine and the other to the supporting frame of the vehicle. In addition, one of the connector members, along with the elastomer element, forms a housing for a fluid dampening means which includes a chamber filled with a liquid, a flexible diaphragm forming one wall of the chamber adjacent the connector member, and an inertia track within the chamber between the elastomer element and the diaphragm and separating the liquid within the chamber. The inertia track has an aperture or orifice therein which permits the flow of liquid through the aperture from one side of the inertia track to the other in response to movement of the elastomer element under load. Substantial additional dampening of vibration and cushioning of light shock loads is provided by the fluid cushion in conjunction with the solid elastomer.
However, attempts to apply automotive fluid mount technology directly to marine applications were unsuccessful. Automotive fluid mounts were selected for the two lateral front mounts on an inboard marine engine based on automotive criteria related to engine size, mount loading, and mount location. This included providing an inertia track within the fluid chamber with an aperture sized for automotive application. There was found to be no improvement in vibration isolation nor was there an improvement in the dampening of low shock loads as a result of normal wave action. More significantly, however, automotive fluid mounts operated totally unsatisfactorily under high shock loads, as might typically be imposed as a result of heavy wave action where the boat drops vertically from the crest to the trough of a large wave.
Under such circumstances of operation the load imposed on a marine mounting system is often as high as 20 g's and peak loads as high as 40 g's have been encountered with larger engines. Under these conditions, conventional automotive fluid mounts bottom out and quickly fail. By comparison, the fluid mounts in an automotive application experience loads of only 5 to 6 g's in normal operation. When these mounts are used in a marine application under high shock load conditions, they bottom out and quickly fail, either through failure of the elastomer or the casting comprising one of the rigid connecting members.